1. Field of the Invention
The present invention relates to an apparatus for creating an artificial target for presentation to an associated radar system under test, but more specifically, it relates to a shipboard sensor exerciser apparatus wherein target senario parameters such as range, radar cross section, azimuth, velocity, and elevation are under manual control of an operator.
2. Description of the Prior Art
A number of different systems can be found in the prior art which met the requirement of producing a return signal to a radar system under test which simulates an artificial target. For proper system operation in producing the artificial target, the return signal must be an appropriately attenuated delayed and possibly phase shifted version of the radar transmitted signal, i.e., the RF burst signal. Prior art systems have been designed to meet the foregoing requirements, but not without attendant problems.
In the prior art, one technique for creating delays in RF signals is by using surface acoustic wave delay lines. These type of delay lines can be used to actually delay the RF signal in real time. However, these type of delay lines are unwieldly. For example, to simulate a delay associated with a target ten miles away, a length of line five miles long would be required. Thus, there is a need in the prior art, in the configuring of apparatus for creating an artificial target to be able to create delays in the RF signal in real time, but yet configure the apparatus to have the facility of portability.
Another technique by which RF signals can be delayed is by using a frequency memory loop. As it is known in the prior art, a frequency memory loop comprises a delay line in series with an RF amplifier whose output terminal is tied back to the input terminal of the delay line. In this closed loop configuration, the gain of the RF amplifier is set to just compensate for the losses of the delay line. Hence, an RF signal could be made to circulate in the loop indefinitely before it is routed to an output port. Each time the RF signal circulates once corresponds to a delay time equal to the design delay of the delay line. Theoretically, an indefinite number of circulations would mean an indefinitely long delay time. However, in practice, there are limitations to the number of times an RF signal can be circulated and still maintain spectral purity, i.e., not be corrupted by signal to noise degradation. Because of the foregoing, the frequency memory loop has a practical maximum delay value of only around 15 microseconds, which corresponds to a simulated target range of only 7,500 feet. In actual practice, simulated target ranges are usually in the order of 10 miles to hundreds of miles. Consequently, there is a need in the prior art to simulate target ranges in the order of 10 miles to hundreds of miles while maintaining signal to noise integrity of the RF signal.
Still another technique involves frequency translating the RF signal down to a convenient intermediate frequency and then using an intermediate frequency delay line to create the desired delay. A final step would be frequency translating the delayed intermediate frequency signal back to the RF range to produce a delayed RF signal. The problems with this technique are similar to those associated with the techniques previously mentioned plus the added problems of increased costs and decreased reliability which are both related to the increased circuitry involved. Hence, there is a need in the prior art to create desired delays in the RF signal while lowering system costs and increasing system reliability.
The prior art, as indicated hereinabove, disclose what could be called advances in radar target simulators. However, insofar as can be determined, no prior art radar target simulator apparatus incorporates all of the features and advantages of the present invention.